Radiotherapy is the use of highly ionizing radiation to damage or kill diseased (e.g. cancerous) cells in a patient. In one form of radiotherapy, a linear accelerator is employed to accelerate charged particles either towards the patient (in which case the charged particles themselves act as the therapeutic radiation) or towards an x-ray target (in which case the x-rays so generated have the therapeutic effect). The radiation is highly damaging to all cells in its path (i.e. both healthy and unhealthy), so the shape and direction of the radiation must be carefully controlled to conform to precise standards.
As part of this process, the radiation is usually first collimated into a beam (cone- and fan-shaped beams are well known in the art, but other shapes are possible) by so-called “primary” collimators, and then further collimated to conform to the shape of a target region within the patient by one or more secondary collimators (e.g. multi-leaf collimators). Considerable research and effort is spent in ensuring the radiation is as closely focussed on the target region as possible, so that damage to the surrounding healthy tissue is minimized. For example, it is known to integrate imaging systems with radiotherapy to provide real-time feedback of the target position. However, the benefits of such systems are significantly reduced if the radiation beam itself is incorrectly calibrated from the start.
When a linear accelerator (linac) is set up, the beam position needs to be measured to prove that it meets the PTS (Production Test Schedule) specification. Conventionally, this is done using a single ion chamber that is scanned through the beam. The chamber is aligned by eye to the projection of a cross wire which had itself been aligned (by eye) to the centre of the beam. This method has an inherent build-up of tolerances of ˜2 mm (for example in the backlash of the scanning movement, and in the alignment by eye).
It is an object of the present invention to provide a more accurate, repeatable method of calibrating the linear accelerator.